The present invention relates to a method for producing a halftone dot in a halftone plate picture image from an original picture having a continuous tone required for manufacturing printings, and more particularly relates to a dot generator which forms an individual halftone dot by collecting a plurality of minute dots.
Methods of such types described above have been known by the Japanese Patent Publication Nos. 52-33523 and 52-49361, particularly the latter disclosing a method in which a screen angle of a halftone plate is taken rational tangent is adopted to avoid occuring mutual moire-image interference fringe among four color plates of cyan, magenta, yellow and black for multi-color printing and each of the screen angles is different one another.
FIG. 1 shows an example which adopts a screen angle of tan .theta.=1/3corresponding to 15.degree., and other three screen angles are tan .theta.=0, tan .theta.=1/1, and tan .theta.=3/1 which do not generate moire-image interference fringes.
They are screen patterns of which screen angles of FIG. 1a, 1b, 1c, and 1d are corresponding to 0.degree., 15.degree., 45.degree. and 75.degree. respectively, and of which lengths of the longitudinal and lateral sides are equal.
In the above method halftone dots may be output on the entire picture image by means of referencing addresses which incliment sequentially in the main scanning direction by a memory device of relative small capacity.
That is, any of directions of longitudinal and lateral side of the memory construction of the square or the rectangular image in which the screen pattern is written agrees with the direction in which the addresses are referenced. Additionally, in general, there occurs no case that the address-referencing is performed passing over or applies data at the same coordinates position twice.
In this method, to produce a halftone dot having a screen angle of 15.degree. a memory device having a capacity of covering relative wide area including more than several halftone dots therein (examples shown in FIG. 1b and 1d are cases of ten halftone dots) is provided and a diagonal screen pattern is written therein.
As easily understood from FIGS. 1a to 1d, each of those screen pattern written in this memory device may be connected continuously with it opposite sides.
However, in case that a screen patterns for multi-color printing are unable to take screen angles of 0.degree., 15.degree., 45.degree. and 75.degree. exactly due to a certain restriction of printing, content of an original picture, etc., a moire-image interference fringe may be produced.
In order to eliminate such moire-image interference fringe, the number of screen line needs varying, e.g., the number of minute dot included in a halftone dot needs varying while using different screen pattern, or the diameter of the minute dot or the distance between minutes dots needs varying while using same screen pattern.
In the former method in which the screen pattern is varied, providing a set of four color screen patterns corresponding to variety of screen number is very troublesome.
Therefore, it is further convenient to apply the latter method in which the diameter of the minute dot or the distance between minute dots may be varied by using a zoom lens or an interchangeable lens in addition to varying the scanning pitch or the scanning clock in using the same screen pattern.
Particularly, alteration of the scanning pitch corresponding to the number of screen line brings disadvantages such as the scanning mechanism becomes complicated, adjustments of the distance between the light beams, the diameter and the intensity thereof, etc. are required. Additionally, another disadvantage is that a relationship between a coordinate system in which character, ruled line, etc. are quantized and a coordinate system in which pictorial pattern is quantized can not be held constant, thus a picture containing both character and the like and pictorial pattern can not be recorded at the same time in scanning. Moreover, it is impossible to record a plural set of halftone picture plates in a same size having different number of scanning lines based on a same picture informations stored in a recording means such as magnetic disc etc.
As one of the data which suggests a method improving the above-described disadvantages, there is a treatise "New Development in Scanner Technology" described on page 251 of the "Progress of Technical Association of the Graphic Arts" published in 1981.
Here, the treatise discloses a method in which a square memory pattern of which a side in length a fundamental period of a halftone dot is referenced in an oblique direction which is not required to agree with a direction of a side of a screen pattern.
With respect to the above-described method a brief description will be given hereinafter.
FIG. 2 shows a square screen pattern representing information of a halftone dot, of which one side is a fundamental period of the halftone dot.
FIG. 3 is a view illustrating coordinate transformation in case that the square screen pattern memory in which said screen pattern is written is referenced sequentially in a oblique direction.
Directions of X and Y axes are coordinate axes of address of said memory, agreeing with a direction of fundamental period of a halftone dot. u and v denotes a scanning direction and a scanning pitch direction respectively.
As shown in FIG. 3, assuming that an angle between the scanning direction and the X axis of the coordinate system is .theta., the following equation of coordinate transformation are established, that is; EQU X=u cos .theta.-v sin .theta. EQU Y=u sin .theta.+v cos .theta. (1)
here, assuming that the interval between the minute dots forming a halftone dot is p, and putting u=mp, v=np, then, the above equations become as follows; EQU X=mp cos .theta.-np sin .theta. EQU Y=mp sin .theta.+np cos .theta. (2)
Assuming that the coordinates of the subsidiary scanning are constant during one period of the main scanning operation, the afore-described equations become as follows; EQU X=mp sin .theta.+C.sub.1 EQU Y=mp sin .theta.+C.sub.2 ( 3)
where, EQU C.sub.1 =-np sin .theta. EQU C.sub.2 =-np cos .theta.tm (3')
The latter equations (3') do not vary during one period of the main scanning operation.
At each of the beginning points of the respective main scanning operation, C.sub.1 and C.sub.2 are previously calculated to be set as X=C.sub. and Y=C.sub.2, and from which every time the main scanning operation advances and interval of a space between each of the minute points for forming a halftone dot, if p cos .theta. is added to an addresses of the X coordinate and p sin .theta. to that of the Y coordinate, addresses to which each of screen patterns at respective times can be referenced.
In this case if each of the addresses of X coordinate and that of Y coordinate of the afore-mentioned screen pattern are made to Nth power of 2, in the course of address computation of the above described expression 3 by basing upon binary rotation, even if there occurs overflow to carry in the computation, the screen pattern memory can shift endlessly from the left to the right vice versa by neglecting the carried portion.
In this case if N is 6 and a screen pattern of more than 256 gradations by 64.times.64 addresses is used, a sufficiently smooth screen pattern can be obtained. However, in the case of accuracy of caluculation being not high, even if it is desired to set screen angle .theta.=15.degree., when p cos .theta. and p sin .theta. are calculated, because of rounded errors caused by digital calculation, the same repetition occurs relatively in short time interval.
Accordingly, in order to avoid the afore-mentioned disadvantage even if one side of the screen pattern memory of square shape is 6 bits (2.sup.6 =64), accuracy of calculation of X and Y coordinates should be extremely high. The accuracy is such as an accuracy of a degree which can sufficiently calculate the number of necessary halftone dots to the size of the largest size that can be output. For example, size of the larger side is 30 inches, the number of screen lines is 175/inch, the number becomes 175.times.30 .times.64=336,000&gt;2.sup.18. Accordingly, if any number larger than 18 bits and multipliers of 1 byte (8 bits) is convenient, an address may be set to a degree of 24 bits or 32 bits.
Whether a dot should be exposed or not can be determined by comparing a screen level obtained by referencing the screen pattern memory basing on X and Y addresses obtained as afore-described with a picture image level. Collection of plurality of thus exposed minute dots form an individual halftone dot.
In the above described method a halftone picture of any described screen angle can be output using the same screen pattern only by changing the screen angle .theta., and further, by varying value of p, a halftone dot of any given screen line number can be output. That is, as p is put smaller, times of adding operation necessary for X and Y addresses to attain to the fundamental period of the screen pattern are increased, so that even if the main scanning clock and the scanning pitch are fixed, a large halftone dot can be output.
Basing upon the above described technique, a feeding mechanism, a main scanning clock generating circuit etc. can be simplified. Further, even if there are any variations in feeding pitch, adjustment and/or modification regarding distance between beams, size of the beams, intensity of the beams etc. which have been necessary in those conventional methods are unnecessary.
However, in this method there is also a serious disadvantage as follows; that is, in this method a screen level D of which coordinates are obtained by using the pitch p which corresponds to a desired screen angle .theta. and the desired screen line number is compared with the picture image level, and according to the result whether exposure should be performed or not is determined. Accordingly, even in the case of an identical contour line configuration being referenced by basing upon the degree of conformity of a contour line configuration corresponding to the picture image level E on the screen pattern to the reference coordinates, there occurs dispersion in the number of exposed minute dots.
FIGS. 4a and 4b show one of the examples of the above described cases. FIG. 4a shows a case in which the degree of conformity is good, and FIG. 4b shows another case in which the degree of conformity is not good. Here in FIGS. 4a and 4b, there are shown cases in which exposing operation is carried out when the screen level D is higher than the picture image level E which corresponds to light and darkness of the pictorial pattern to be reproduced.
As described the above, even if it is identical with the picture image level, depending upon the degree of conformity of the coordinates which refer to the screen pattern, there comes out considerable difference in the number of the minute dots to be exposed. Such dispersion in the number of these exposed minute dots results in occurrence of fatal blurs in the halftone plate for printing.
Of course, a method in which merely the intensity of an exposure light beam is varied stepwisely is disclosed in the specification of the U.S. Pat. No. 4,025,189. The object of the invention disclosed in the patent is not to pursue subtle control of an area of a halftone dot, but lies in consequently forming a halftone dot (soft dot) of which quantity of silver at the peripheral edge portion is less than that of the central portion thereof.
In the plate-making process, when it is desired to make a part or the whole of halftone dots of a film once having been exposed and developed smaller, the dot etching is performed. It is very convenient that in this case even if the halftone dot is made smaller by performing etching, in a soft dot there remains considerable quantity of silver in the central portion.
The method disclosed in the afore-mentioned U.S. Pat. No. 4,025,189 is that when the intensity of an exposure light beam is calculated, if a pair of values aligned in the scanning pitch direction of a screen pattern memory are accessed simultaneously, and differences obtained by comparing each of the values with the level of the same picture image are put as a and b respectively, the results are as follows; that is, if both a and b are positive values, the intensity is 100% of the intensity of the light beam, if both are negative, the intensity is 0% of that of the light beam, and in the case of a and b being different signs, the intensity of the exposure light beam is calculated basing upon the following equation, (a+b)/(a-b)=intensity of exposure light beam.
However, this method is on the premise that in case of multiprintings, the memory referencing method disclosed in the afore-described Japanese Patent Publication No. 52-49361 is applied. Accordingly, there is no description in the publication with respect to excellency in the degree of conformity of the coordinates to which the screen pattern in question is referenced, and dispersion in the number of minute points.
In addition, there is another disadvantage that calculation for determining the intensity of the exposure light beam is quite complicate.